In nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), the RF coil, or probe, used to acquire data is typically tuned to adjust the frequency of the coil and impedance matched to match the impedance of the coil to the system, for optimum performance. The problems associated with tuning and matching NMR/MRI radio frequency (RF) coils have been addressed in many different ways. The usual method is to place a lumped circuit of tuning and matching capacitors in close proximity to the RF coil.
Variable capacitors are preferred in order to adjust the tuning/matching with varying loads, but are physically much larger than fixed capacitors and, thus, sometimes difficult to locate in close proximity to the coil. In some cases it becomes time consuming and difficult to tune and match the coil properly with varying loads and coupling once it is inside the magnet, as the coil is difficult to reach. The common solution to this problem has been to employ mechanical extension arms to reach inside the magnet, mechanically couple to the variable capacitor's adjustment knob, and adjust the variable capacitors from a distance, in order to tune and match the coil. Such mechanical extension arms are time-consuming to use, and are difficult to couple to, and adjust, the variable capacitors. Further, lack of feedback from the mechanical extension arms makes it difficult to accurately determine the position of the variable capacitor's adjustment knob which can lead to broken capacitors.
Another solution is to place the matching circuit outside the magnet at the common point, usually the transmit/receive module. Matching is then realized from this point to the coil. Power transfer to the RF coil from the RF transmitter and the signal detected by the coil and received at the preamp relies on the characteristic impedance of the transmission line. Accordingly, many previous approaches to remote tuning have suffered from instability due to stray capacitances.